This invention relates generally to the treatment of wastewater and deals more particularly with a method and apparatus involving the programmed sequential aeration and/or mixing of multiple wastewater zones one at a time or in programmed combinations in order to achieve energy savings, process control or other benefits.
Conventional treatment of wastewater makes use of biological techniques and commonly involves applying aeration to a biological reactor such as a treatment basin. Aeration promotes the growth of bacteria and other microorganisms which remove soluble organic wastes from the water. The aeration serves the function of supplying oxygen (oxygenation) and also the function of mixing the wastewater so that the aerobic organisms and the soluble organics remain distributed and interact throughout the volume of the water to be treated. Subsequent to the aeration, the organisms may be removed in a clarifier and some may be returned to the biological reactor for use in the continuing treatment process. Excess sludge is removed and disposed of by known techniques. Alternatively, some systems have no or very low biological activity such that clarification may not be required. Nevertheless, mixing is required in order to maintain solids in suspension in the wastewater.
Processes of this type are routinely employed for activated sludge treatment with detention times as short as 1-4 hours. Modifications of the process involve treatments requiring significantly longer detention times that may exceed 48 hours. Currently, the process is employed as a continuous flow process in which the wastewater flows continuously through the treatment basin and the entire basin is continuously aerated.
Although this conventional aeration process is widely employed, it is characterized by substantial energy requirements in order to achieve adequate mixing. In the past, the oxygen transfer has been relatively inefficient, and the air requirements for oxygenation have almost always exceeded the air requirements for mixing. However, more modern diffuser technology provides more efficient oxygen transfer and the mixing requirements can exceed the oxygenation requirements. In many cases where high efficiency diffusers are used, the cfm to meet the oxygen requirements is less than the cfm to meet the mixing requirements, and the aeration rate must be significantly higher than required for oxygenation in order for adequate mixing in the entire basin to result.
By way of example, in a typical treatment system having a capacity of one million gallons per day in an extended aeration application at a BOD level of 200 milligrams per liter, the rate of aeration required for oxygen may be approximately 1550 cfm (cubic feet per minute) based on the use of high efficiency diffusers and a given basin depth. For the same system, biological or solids mixing may require about 2500 cfm of air using present technology and based on the accepted value of 0.12 cfm per square foot of floor area. The aeration capacity for this system using conventional techniques must be 2500 cfm for mixing, and the entire basin must be continuously aerated at this rate even though it is excessive from the standpoint of oxygenation and results in about 950 cfm of air essentially being devoted solely to mixing and not used for oxygenation. Use of high efficiency diffusers causes this waste of energy to increase.
It is thus evident that conventional systems must be equipped with a blower capacity well in excess of the capacity needed for oxygenation and, perhaps even more importantly, require much more energy for mixing than is needed to achieve only oxygenation of the wastewater. Even more of the total energy is devoted to mixing as the BOD level decreases because the oxygenation requirements then decrease with no corresponding decrease in the mixing requirements. High efficiency diffusers create the same effect. Mixing only systems such as equalization basins represent the ultimate in mix limited systems.